Project Abstract VTE affects 900,000 individuals in the US each year, and is highly heritable, with ~60% of lifetime risk attributable to genetic factors. To date only 50% of the genetic factors contributing to VTE have been described. Recently, genome-wide association studies (GWAS) of VTE have validated the contribution of common variants of coagulation factor V (F5) and common blood group antigens (ABO) as well as smaller contributions at six other gene loci, but have failed to identify additional variants implicated by the high heritability of VTE. Due to the lack of a comprehensive understanding of the genetic risk factors for VTE, current genetic testing cannot accurately predict an individual?s susceptibility for VTE and does not adequately inform clinical decision making for VTE prevention and treatment. Therefore, there is a critical need for more complete genetic risk data to help clinicians predict which individuals need treatment to prevent incident or recurrent VTE. The objective of this proposal is to identify new, high impact genetic risk factors for VTE and to begin to uncover the molecular mechanisms affected by newly identified risk variants. The central hypothesis of this proposal is that a large proportion of the unknown genetic risk factors for VTE are rare variants that cluster in a limited number of genes driving risk primarily through loss of function. These yet to be described variants contribute to the heritability of VTE but are undetectable by GWAS because they are individually too rare. To identify mutations that alter VTE risk, collapsing rare variant analyses will be performed with whole-exome sequencing data from a previously collected VTE case/control study. Subsequent targeted DNA sequencing in an extensively characterized young healthy cohort combined with plasma antigen assays will inform the molecular mechanisms leading to a prothrombotic state in individuals harboring VTE risk mutations. Functional studies of the newly identified VTE risk gene STAB2 to identify quantitative or qualitative deficiencies in the VTE case variants will be performed. Additional functional assays will be developed after identification of physiologic stabilin-2 ligands through proximity dependent biotinylation and proteomics. Furthermore, to comprehensively characterize the effect of missense mutations on stabilin-2 function, focused mutagenesis studies of stabillin-2 domains will be performed. Upon successful completion of the proposed research, we expect to have identified previously undescribed gene deficiencies that explain a substantial proportion of the inherited risk for VTE. We also expect to define the basic mechanisms underlying genetic associations in STAB2 and other novel genetic associations. The ability to execute both genome-wide screens and ?wet lab? experimentation is facilitated by the applicant?s established cross- disciplinary collaborations. The innovative use of DNA sequencing and rare variant analyses in well characterized human cohorts, combined with in vitro mutagenesis and proteomic studies will lead to new knowledge that provides reproducible, clinically useful and personalized information for prognosis, prevention, and treatment of VTE.